Polyolefin fibers, and in particular polypropylene fibers, present particular characteristics which have contributed to their development: high chemical inertness, no polar groups, allow specific weight, low heat conductivity, good insulating power, good resistance to abrasion, little power to absorb water, good resistance to mold and to bacteria, and excellent color fastness obtained by including pigments during spinning. Such polyolefin fibers are used on their own or mixed with other fibers in the fabrication of products belonging to the textile sector proper, such as underwear, sports garments, and carpets, or indeed the industrial sector such as engineering fabrics, or filters, or indeed the hygienic/sanitary sector such as sanitary napkins. In this hygienic/sanitary sector, it is the properties of resistance to mold and bacteria, chemical inertness, and poor ability to absorb water that constitute the main advantages of polyolefin fibers, and in particular of polypropylene fibers. These fibers are considered as being non-toxic and non-cytotoxic.
Polypropylene fibers are generally produced using the molten spinning technique which consists in melting the polymer with various additives in an extruder at high temperature, and in causing the resulting molten material to pass through the holes of an extrusion head, a metering pump serving to maintain a constant pressure during this operation. The hot filaments leaving the extrusion head are cooled by a flow of air and are subjected to various operations, in particular to stretching and cutting, in order to obtain fibers having desirable cohesion and mechanical characteristics.
Non-woven fabric is fabricated from polypropylene fibers by various techniques, including heat-bonding. In this technique, the polypropylene fibers are in the form of a sheet and they are compressed while hot so as to obtain inter-fiber bonds by localized surface melting of the fibers in the bonding zones.
Until now, there have been three types of polypropylene fiber that are specially designed to be used in fabricating non-woven fabrics by the heat-bonding technique.
A first type of fiber, disclosed in particular by documents U.S. Pat. No. 4,473,677, U.S. Pat. No. 5,985,193, and WO 99/55942, comprises two-component fibers. They are obtained by using two extruders, each being fed independently with a specific polymer, the two polymers in question having melting temperatures that are slightly different. During the heat-bonding operation, it is the component that has the lower melting temperature that melts in order to provide bonding between the fibers. The drawback of that first type of fiber stems in particular from the relatively high cost of fabrication due to the fact that a special spinning installation is required for the co-extrusion.
In a second type of fiber, as disclosed in particular by documents U.S. Pat. No. 5,985,193 and WO 99/55942, the fibers are likewise two-component fibers made by a polymer mixture obtained directly from a single extruder. The structure of the fibers is irregular, with one of the components forming islands in the other component, and with the dimensions of the islands being a function of the miscibility of the two components.
In a third type of fiber, as disclosed in particular by documents U.S. Pat. No. 5,281,378, U.S. Pat. No. 5,318,735, U.S. Pat. No. 5,431,994, U.S. Pat. No. 5,709,119, U.S. Pat. No. 5,882,662, U.S. Pat. No. 5,985,193, and U.S. Pat. No. 6,116,883, the fibers present a surface skin that is degraded. Such fibers are commonly referred to as “skin” fibers. They are obtained under special conditions in the spinning and cooling process leading to molecular degradation by thermal oxidation at the periphery of the fibers, the skin being constituted by a surface layer of polymer degraded in this way. During the heat-bonding operation, cohesion between the fibers is obtained by melting of the polymer that constitutes the degraded skin, which polymer presents a melting temperature that is lower than that of the non-degraded polymer constituting the fiber core.
From the above examination of the three types of polypropylene fiber that has been specially designed for use in fabricating non-woven fabrics by heat-bonding, it can be seen that cohesion between the fibers is always obtained by means of a polymer portion situated at the surface of the fiber that presents a melting temperature lower than the melting temperature of the polymer constituting the remainder of the fiber. The polymer portion having a lower melting temperature enables inter-fiber bonds to be created in the non-woven fabric, while the remainder of the polymer which is not directly involved by the heat-bonding serves to maintain the mechanical properties of the fibers.
Concerning the costs of fabricating the three above-mentioned types of fiber, there is no doubt that degraded skin fibers are the best since they do not require a special installation for fabrication purpose, merely requiring special conditions to be implemented during spinning and cooling.
Nevertheless, degraded skin fibers present certain drawbacks. Although no clear explanation is available for this phenomenon, it is observed that degraded skin fibers are less soft to the touch, presenting a harder “handle” or “feel”. This discomfort is further increased when accessing the software of a non-woven fabric obtained with such degraded skin fibers, since the inter-fiber bond zones that have been melted are themselves rigid, and are made of the degraded polymer.
Furthermore, in order to obtain a degraded skin during spinning, the Applicant has found it necessary to work under operating conditions that are precise, and thus with an operating window that is quite narrow. The difficulty of controlling such operating conditions leads to frequent variations in the quality of the non-woven fabric obtained using such fibers.
Finally, in order to obtain molecular degradation by thermal oxidation it is necessary to work at a high extrusion temperature which leads to greater energy consumption.